There has been a desire in recent years for flexible electronic devices which in turn has driven a need for flexible electronic components that can be applied to flexible (polymer) substrates at low temperatures. Although polymer and organic light emitting diodes are generally available, reliable, easy to process polymer diodes are not readily available for flexible electronics applications. Some work has been done with wet electrolytic systems. Both solutions require good sealing for long lifetimes. There has been a movement to use high speed. printing and other deposition methods rather than subtractive lithographic methods.
Approaches have generally centered on solution processable semiconductors such as those used in polymer light emitting diodes (PLEDs) sandwiched between electrodes that can be deposited, and optionally sintered, at low temperatures. Many of the semiconducting materials are difficult to process and can have lifetime issues. Some may chemically de-dope and become inactive. In addition, such materials can be sensitive to atmospheric moisture and need to be sealed. Multilayer structures may be difficult to fabricate. Many of these electrode systems require sintering temperatures that can cause damage to the polymer substrate and need to be tailored to have the correct work function for diode operation. Some methods have introduced pressure-annealing or lamination steps to improve the performance of the devices.
For example, Yoshida et al., in Jpn. Appl. Phys. 50 (2011) 04DK16 describe a pressure-annealing method for fabricating printed low-work-function metal patterns and printed metal alloy patterns. The pressure-annealed metal electrodes of Yoshida et al., are used as bottom electrodes of printed polymer diodes.
Reports on the development of solution-state polymer diodes with nanogap electrodes that support intra-chain-dominant conduction are provided at http://nanotechweb.org/cws/article/lab/50114.
In all these cases, the diode mechanism has moving charges—electrons and holes—which flow through an electrically (semi-)conductive layer with similar mobilities (relative to the mobilities of any of the molecular species in the layer between the electrodes). Ionic diodes are known in the art but typically these require fluidic electrolytes to enable ionic mobility.
Lee et al., in U.S. Published Patent Application Nos. 2007/0221926 and 20120025174 describe the production of solution-processed titanium oxide layer containing polymer diodes.
There continues to be a need in the art for polymer diodes that are suitable for use in flexible electronics applications.